I second this defect. Subtree support is experimental, at best - it is extremely fragile, and not fit for production use.
1. Subtrees added from the command line are not handled correctly by SourceTree. The subtree remote (if added) is treated as a normal remote, with no knowledge about the required "--prefix" on push or pull. Also, SourceTree quickly hangs when updating the log for this local repo.
2. Subtrees added from SourceTree seem to work decently within the local repo. If the subtree commits are pushed to a remote, and that remote is cloned, SourceTree hangs trying to display the log in the new local repo.
Subtrees is the only current git "module/component" solution we can live with, and the way SourceTree is broken in this respect, basically means that SourceTree is unusable to us.
Please update to the latest version of Sourcetree.
This issue is no longer occurring.